The present invention relates to a mark for position detection and a mark detecting method and apparatus, and also relates to an exposure system. More particularly, the present invention relates to a mark for position detection formed on a plane surface of a substrate and a method and apparatus for detecting the mark, and also relates to an exposure system having the mark detecting apparatus as a device for detecting the position of the substrate. The mark for position detection according to the present invention is suitable for use in an exposure system to align a mask pattern and a photosensitive substrate in a photolithography process in which the substrate is exposed in accordance with the mask pattern to produce, for example, a semiconductor device.
In photolithography processes for producing micro devices, e.g. semiconductor devices, liquid-crystal display devices, image pickup devices (CCDs), or thin-film magnetic heads, a projection exposure system is used in which an image of a photomask or a reticle (hereinafter generally referred to as "reticle"), which has a transfer pattern formed thereon, is projected onto a substrate coated with a photosensitive material (photoresist), e.g. a wafer or a glass plate (hereinafter referred to as "wafer"), through a projection optical system.
In this type of projection exposure system, alignment of the reticle and the wafer must be carried out with high accuracy before exposure. To perform the alignment, the wafer has marks for position detection (alignment marks) formed (transferred by exposure) in a preceding photolithography process. By detecting the position of such an alignment mark, the position of the wafer (or a circuit pattern on the wafer) can be accurately detected.
The alignment marks on the wafer are entirely unnecessary for the operating characteristics of the completed micro device. Therefore, it is desirable that the size of the marks be as small as possible. Alignment marks are generally set in the boundary regions between micro devices, which are known as "street lines", i.e. "margins" for cutting the micro devices from each other after the completion of various processes. The street lines are belt-shaped regions each having a width of the order of from 70 to 90 micrometers. Therefore, the length of each shorter side of alignment marks is desirably not greater than 70 micrometers.
Some methods of detecting the position of a mark on a wafer have already been put to practical use. The mainstream of recent mark position detecting methods is an image detection method in which an optical mark image is detected, and the mark position is detected on the basis of the image intensity distribution.
Because the above-described alignment requires an extremely high degree of accuracy, the mainstream of conventional mark position detecting methods is such that marks (X mark and Y mark) are used exclusively for two orthogonal directions (X direction and Y direction), and the positions of these marks are successively measured. In general, line-and-space patterns having periodicity in the measuring directions have heretofore been used as the X and Y marks (marks for one-dimensional measurement).
FIGS. 8A and 8B show examples of the conventional marks for one-dimensional measurement. FIG. 8A shows a mark MX used for detection in the X direction. FIG. 8B shows a mark MY for detection in the Y direction. These marks MX and MY are used in a pair (there has been no specific restriction on the positional relationship between the two marks). In the conventional practice, position detection is first carried out with respect to one direction (X direction or Y direction) using one mark, and then position detection is carried out with respect to the other direction (Y direction or X direction) using the other mark.
As has been stated above, the length of each shorter side of these marks is demanded to be not greater than the street line width (in general, from 70 micrometers to 90 micrometers). Therefore, the widths of these marks in the non-detecting direction (Y direction for MX; X direction for MY) have generally been restricted to a length not greater than about 70 micrometers from the above restriction.
A mark detecting optical system for detecting the position of a mark as described above needs to be corrected for aberrations to an extremely high degree. The aberration correction includes not only the correction of aberrations due to errors in designing the optical system but also the correction of aberrations due to errors in machining, i.e. decentration of the lens and an error in the surface accuracy. In particular, errors in machining are difficult to eliminate completely. Therefore, after the assembly of the optical system, adjustment is made to minimize the residual aberrations at a specific "mark detecting position" (one point in the detecting area near the optical axis of the detecting optical system), thereby reducing the influence of the residual aberrations during the detection. Accordingly, if the position for detecting a mark deviates from the above-described minimal aberration point, a detection error due to the residual aberrations arises, and the mark detection accuracy degrades.
In the above-described conventional technique, the mark detection for position detection is carried out with respect to the X and Y directions separately from each other. Therefore, it takes a long time to detect the marks, and this causes the processing capacity (throughput) of the projection exposure system to be reduced unfavorably.
In view of the above-described circumstances, a conventional technique uses two-dimensional marks that enable simultaneous detection with respect to both the X and Y two-dimensional directions.
FIG. 9 shows one example of two-dimensional marks for detection in both the X and Y directions. A mark MG shown in FIG. 9 per se has periodicity in two-dimensional directions. However, as will be clear from FIG. 9, the size of the mark edge (boundary between black and white), which is effective for the position detection in each of the X and Y directions, relative to the mark area undesirably reduces to approximately a half of that of one-dimensional marks (MX and MY) because of the periodicity in the two-dimensional directions. Therefore, the mark area must be increased in order to obtain a detection accuracy equal to that in the case of the one-dimensional marks (MX and MY). However, if the mark area is increased, the length of one side (or shorter side) of the mark becomes greater than the street line width (e.g. 100 micrometers or more in the case of FIG. 9), and hence a part of the mark undesirably extends over the circuit pattern on the wafer. Accordingly, the restriction on the mark formation position increases unfavorably.
If a two-dimensional mark Mt as shown in FIG. 10 is used in which an X-direction one-dimensional detection mark portion Ma and a Y-direction one-dimensional detection mark portion Mb are disposed in a side-by-side relation to each other, the length of each shorter side of the mark can be made not greater than the street line width.
However, the mark Mt shown in FIG. 10 suffers from problems in terms of the detection accuracy. That is, the mark detecting optical system has been adjusted such that the residual aberrations are minimized at a specific "mark detecting position" (one point in the detecting area near the optical axis of the detecting optical system), as stated above. Therefore, if a detection mark portion for one direction, e.g. the Y-direction detection mark portion Mb (or the X-direction detection mark portion Ma), is disposed near the optical axis, the other mark portion Ma (or the mark portion Mb) for the other direction lies apart from the optical axis. Consequently, the detected value for the position of the latter mark portion Ma (or Mb) is adversely affected by the residual aberrations of the optical system when the position of the mark Mt is detected with respect to both the X and Y directions simultaneously. Consequently, detection errors increase unfavorably. (This problem will be described later in more detail to compare the present invention with the conventional technique in the description of the embodiments).